Abstract Duringthedevelopmentofheartfailurecardiacfuelmetabolismswitchesfrompredominantlyfattyacidoxidation (FAO)toincreasedrelianceonglucose,especiallyglycolysis.Thismetabolicremodelingisgenerallyrecognized and considered ultimately maladaptive for sustaining myocardial energetics and function. The mechanisms responsiblefortheswitcharepoorlyunderstoodbutappeartobecoupledwithimpairedmitochondrialfunction. Downregulations of multiple transcriptional mechanisms, such as PPARa? or PGC-1a?, for FAO have been identified in heart failure. Reduced FAO could release the inhibition of glucose use through Randle cycle and thuspromotemyocardialglucoseutilization.However,thishypothesisdoesnotexplainwhyreducedFAOleads to predominantly glycolysis uncoupled with glucose oxidation, a phenomenon similar to Warburg effect. In additiontodecreasedFAO,multipleaspectsofmitochondrialfunction,inparticular,oxidativephosphorylation, oxidativestress,andredoxbalance,arealsoalteredinheartswithpathologicalhypertrophy.Theseobservations raise an intriguing possibility that increased glycolysis is driven by mitochondrial dysfunction although the molecularmediator(s)intheswitchareelusive.Recently,wefoundthattheexpressionofmitochondrialATPase inhibitorfactor1(ATPIF1)wasincreasedinrodentheartsorcardiomyocytes(CMs)withpathologicalhypertrophy. UpregulationofATPIF1innon-cardiomyocyteshasbeenshowntoincreaseglycolysis,totriggermitochondrial hyperpolarization and increase the production of mitochondrial reactive oxygen species (mtROS). In our preliminary study, ATPIF1 overexpression also shifted energy metabolism from mitochondrial oxidation to glycolysisinCMs.Therefore,weaskedwhetherandhowATPIF1connectsmitochondrialfunctionandglycolysis intheheartundergoingpathologicalhypertrophy.TheATPIF1iswellconservedfromyeasttohuman,anditis knowntoinhibitthereversedoperationofFoF1-ATPaseinComplexV(normallyfunctionsasATPsynthase)to hydrolyzeATPandthusmaintaintheprotongradientduringreducedmembranepotential,suchasischemia.The consequence ofATPIF1 upregulation in the non-ischemic heart is unknown. In the proposed study, we will determinetheinteractionofATPIF1withComplexVundernormalandstressconditionsandtestthehypothesis that increased ATPIF1 inhibits ATP synthase and triggers the metabolic switch to glycolysis via stimulationofHIF1a?signalingduringpathologicalhypertrophy.Wehavegeneratedpreliminarydataand research tools for the following three specific aims: 1) To test the hypothesis that up-regulation ofATPIF1 increasesmyocardialglycolysisthroughenhancingtheHIF1?signaling.2)Todeterminethemolecular interaction of ATPIF1 and FoF1-ATPase and changes of mitochondrial protein interactome under physiological and pathological conditions using quantitative Protein Interaction Reporter (PIR) technology. 3) To determine the in vivo role ofATPIF1 in the metabolic reprogramming and cardiac responsetostress.